Use MOUSE_FILTER_PASS for all containers (but PanelContainer)

Containers are meant to forward mouse input to their the Controls
they contain.

PanelContainer has a visible Panel stylebox, so it still defaults
to STOP.

Fixes #34933.

scene/gui/container.cpp

@@ -197,4 +197,6 @@ void Container::_bind_methods() {
 Container::Container() {
 
 	pending_sort = false;
+	// All containers should let mouse events pass by default.
+	set_mouse_filter(MOUSE_FILTER_PASS);
 }